Liquid crystal panels for use in liquid crystal display devices, for example, do not emit light by themselves, and thus need backlight units as separate lighting devices. The backlight units are well known for being mounted on the back surface of a liquid crystal panel (opposite to a display surface), and are configured to include a chassis having an opening on the surface of the liquid crystal panel side; a large number of light sources (for example, cold cathode tubes) stored as lamps in the chassis; an optical member (diffuser plate or the like) that is disposed in the opening of the chassis and through which light emitted from the light sources efficiently exit toward the liquid crystal panel; and a reflection sheet that is laid in the chassis and reflects light from the light sources toward the optical member and the liquid crystal panel.
If the light sources emit linear light, the backlight unit is configured to convert linear light by the optical member into planar light, thereby achieving even illumination light. However, if the conversion into planar light is not sufficient, a stripe lamp image according to alignment of the light sources is generated to deteriorate a liquid crystal display device in display quality.
To achieve even illumination light at the backlight unit, it is desired to increase the number of light sources arranged, thereby to shorten a distance between the adjacent light sources or raise the degree of diffusion by the diffuser plate, for example. However, increasing the number of light sources leads to increase in cost and power consumption of the backlight unit. In addition, if the degree of diffusion by the diffuser plate is made higher, it is not possible to raise brightness, which causes the need to increase the number of light sources. To solve the problem, there is a known backlight unit that maintains even brightness while suppressing power consumption, as is disclosed in Patent Document 1.
The backlight unit disclosed in Patent Document 1 includes a diffuser plate disposed on the light output side of a plurality of light sources. The diffuser plate has a light-controlling dot pattern printed with an all-ray transmittance (opening ratio) of 62 to 71% and a haze of 90 to 99%. In particular, the dot pattern is configured such that the diameter of the dots is larger immediately above the light sources and is smaller with increasing distance from the light sources. According to the foregoing configuration, it is possible to efficiently use light emitted from the light sources to radiate light with a sufficient and even brightness value without increasing power consumption of the light sources.
Patent Document 1: Japanese Unexamined Patent Publication No. 2005-117023